The size evolution of passive galaxies : observations from the Wide-Field Camera 3 Early Release Science program

Abstract

We present the size evolution of passively evolving galaxies at z 2 identified in Wide-Field Camera 3 imaging from the Early Release Science program. Our sample was constructed using an analog to the passive BzK galaxy selection criterion, which isolates galaxies with little or no ongoing star formation at z ≳ 1.5. We identify 30 galaxies in 40arcmin to H <25mag. By fitting the 10-band Hubble Space Telescope photometry from 0.22 μm ≲ λ ≲ 1.6 μm with stellar population synthesis models, we simultaneously determine photometric redshift, stellar mass, and a bevy of other population parameters. Based on the six galaxies with published spectroscopic redshifts, we estimate a typical redshift uncertainty of 0.033(1 + z). We determine effective radii from Sérsic profile fits to the H-band image using an empirical point-spread function. By supplementing our data with published samples, we propose a mass-dependent size evolution model for passively evolving galaxies, where the most massive galaxies (M 10 M) undergo the strongest evolution from z 2 to the present. Parameterizing the size evolution as (1 + z) , we find a tentative scaling of α (- 0.6 0.7) + (0.9 0.4)log (M /10 M), where the relatively large uncertainties reflect the poor sampling in stellar mass due to the low numbers of high-redshift systems. We discuss the implications of this result for the redshift evolution of the M -R relation for red galaxies.